What a hardiness zone measures
A USDA plant hardiness zone is built from a single number: the average coldest night of the year. For the 2023 map, USDA and Oregon State University’s PRISM Climate Group took the lowest temperature recorded each winter from 1991 to 2020 and averaged those 30 lows. That average decides the zone.
The scale runs from zone 1 (colder than −50°F, parts of interior Alaska) to zone 13 (above 60°F, the warmest parts of Hawaii and Puerto Rico). Each zone spans 10°F and splits into an “a” half and a warmer “b” half of 5°F each. Zone 7a, for example, means the coldest night of an average winter bottoms out somewhere from 0 to 5°F. Most of the lower 48 states fall between zones 3 and 10.
Nursery tags and seed catalogs print a zone range such as “zones 5–9”. The first number is the cold limit: the plant should survive an average winter in zone 5 and anything warmer. The second number is a rough heat or humidity limit, which the cold map does not measure.
Your zone is not your frost date
This is the most common mix-up in gardening advice. A hardiness zone says how cold winter gets. It says nothing about when the last spring frost happens, and that date is what decides when to plant tomatoes, beans and most other vegetables.
Two places in the same zone can have very different springs. In our data, Boise, Idaho and Louisville, Kentucky are both in zone 7a, yet the average last frost is May 17 in Boise and March 31 in Louisville, about 7 weeks apart. Eugene, Oregon and Dallas, Texas share zone 8b, with last frosts around April 11 and March 2. A planting chart that says “zone 7: plant tomatoes in April” is right for one of those cities and wrong for the other.
So use your zone for perennials and your frost dates for annuals. The tool above shows both, and the frost dates page adds the odds of a late frost. For crop-by-crop dates, open the planting calendar or the seed starting calculator.
What your zone is good for
- Perennials, shrubs, trees and bulbs. Anything that stays in the ground over winter: fruit trees, berry bushes, asparagus, rhubarb, perennial herbs, tulips and garlic.
- Perennial herbs in a vegetable bed. Chives, mint and strawberries come back almost anywhere in the lower 48. Thyme, sage and oregano are reliable from about zone 5. Rosemary is the one that catches people out: it is usually hardy only in zone 7 and warmer. See each plant’s guide, for example rosemary or thyme.
- Choosing varieties. Strawberry, garlic and fruit tree varieties are often bred for colder or milder winters. Garlic is a good example: hardneck types handle cold winters well, softneck types suit milder ones.
- Overwintering crops. Kale, spinach and garlic sail through a zone 7 winter under a little mulch. In zone 4 they need a cold frame or heavy cover.
What your zone leaves out
The map measures one cold night. It doesn’t measure summer heat, humidity, rainfall, soil, wind, or how long the cold lasts. Snow cover matters too: a steady blanket of snow insulates roots, so a plant can survive a snowy zone 4 winter better than a bare, windy one. Summer heat has its own map, the American Horticultural Society Plant Heat Zone Map, which is why some plants rated “zones 5–9” struggle in hot, humid parts of the Southeast.
Your own yard matters as well. Cold air drains downhill and pools in low spots, so a hollow can run half a zone colder than the slope above it. A south-facing brick wall or a sheltered courtyard can be half a zone warmer. Cities hold heat at night. And plants in above-ground containers lose the protection of the soil, so treat potted perennials as a zone or two less hardy than the label says, or move them to a garage for winter.
Why your zone may have changed in 2023
The 2023 map replaced the 2012 version. It uses a newer 30-year period (1991–2020) and many more weather stations, and USDA reports that about half of the country moved into the next warmer half-zone, while the rest stayed the same. A shift of half a zone means the average coldest night is up to 5°F warmer than the old map said.
Treat a new, warmer zone with some caution. The zone is an average: a winter much colder than average still arrives now and then, and it is those winters that kill borderline plants. If a shrub or herb is only just hardy in your new zone, give it a sheltered spot and a thick layer of mulch.
How we look up your zone
The zone comes from the 2023 USDA Plant Hardiness Zone Map data, matched to your ZIP code. A ZIP code can cover hills and valleys, so treat the result as a good estimate; if you are close to a zone boundary, check the official interactive map at planthardiness.ars.usda.gov. Frost dates come from NOAA’s 1991–2020 U.S. Climate Normals for the nearest of more than 7,000 weather stations. The colour scale on this page is our own design, not a copy of the USDA map.
Ready to plan? Draw your beds in the garden planner, check spacing with the square foot gardening chart, and see what grows well together in the companion planting chart.